It is known to provide a variable displacement pump with a pressure compensator for operating a pump adjusting mechanism to vary the displacement of the pump and a spring biased piston which yieldingly opposes the pressure compensator. It is aso known to provide a load sensing valve which functions to vary the displacement of the pump and a same spring biased piston which yieldingly opposes the load sensing compensator valve. The load sensing compensator is active as the primary pump control except when the pressure of the pump is at the limit set by the pressure compensator at which time the pressure compensator overrides the load sensing compensator. In addition, it is known to apply the fluid pressure delivered to a system or directional valve to work in unison with the spring biased piston.
One of the problems with such a system is that under heavy load and when the connecting lines are short, the system tends to become unstable causing hydraulic pulsations and resultant oscillating movement of the load or device being controlled.
Accordingly, an objective of the present invention is to provide a variable displacement pump system which is more stable.
In accordance with the invention, the load pressure as taken from a load sensing port or the system valve is connected to deliver this load pressure to the spring biased piston rather than using the output pressure of the pump to the spring biasing piston.